Method of forming optical thin film

ABSTRACT

A method for forming an optical thin film used for optical elements of laser systems including high-energy lasers and an optical element of optical apparatuses is provided. The optical thin film can be easily formed on a desired substrate with reproducibility by vapor-depositing a porous fluoride layer for preventing reflection in the deep ultraviolet region, and can be easily removed in a short time to reuse the substrate if the thin film damaged.  
     A water-insoluble material ( 2 ) for preventing reflection is vapor-deposited onto an optical element substrate ( 1 ). A water-soluble material ( 3 ) having a higher particle energy is vapor-deposited onto the surface of the water-insoluble material ( 2 ). The water-soluble material ( 3 ) permeates deep into the water-insoluble material ( 2 ) to form a mixed film on the surface of the substrate ( 1 ). Then, the water-soluble material ( 3 ) is dissolved and removed to form a porous thin film ( 5 ) comprising the water-insoluble material ( 2 ).

TECHNICAL FIELD

[0001] The present invention relates to a method for forming an opticalthin film used as an antireflection film of an optical element used forhigh-energy lasers, a thin film for enhancing the laser resistance ofpolarizers and reflectors used for high-energy lasers, and anantireflection film formed on an eye-protection filter for lesseningscreen glare of a display.

BACKGROUND ART

[0002] Methods for forming the optical thin film are roughly classifiedinto vapor deposition and chemical processes.

[0003] Antireflection films formed by vapor deposition include monolayerfilms and multilayered films. The monolayer film is formed byvapor-depositing, on a substrate, a material having a refractive indexlower than that of the substrate at a thickness of one fourth awavelength. In the multilayered film, at least two layers arevapor-deposited using materials having a high refractive index and a lowrefractive index.

[0004] The inventors of the present invention have proposed a methodusing both the vapor deposition and a chemical process. In this method,a water-soluble material and a water-insoluble material arevapor-deposited at one time onto a substrate to form a mixed film, andthen the water-soluble material is removed from the mixed film to form aporous thin film comprising the water-insoluble material.

[0005] The method for forming the porous thin film has been disclosed inJapanese Patent Application Publication No. 5-52923.

[0006] Milam and others of Lawrence Livermore National Laboratory in theU.S. have developed a method for forming an antireflection film using achemical process and in which a porous silica thin film having areflectance of 0.1% to 0.3% is formed on the surface of a quarts glasssubstrate by a sol-gel process. This method has been disclosed in D.Milam et al., CLEO'84 Technical Digest, THB 2 (1984).

[0007] Thomas of the same laboratory has formed porous MgF₂ and CaF₂fluoride thin films on quartz glass and CaF₂ crystal substrates by asol-gel process. This method for forming the porous MgF₂ and CaF₂ filmshas been disclosed in Ian M. Thomas, Appl. Opt., Vol. 27, No. 16, pp.3356-3358 (1988).

DISCLOSURE OF INVENTION

[0008] However, these methods described above have the followingproblems.

[0009] Optical films formed by the known vapor deposition are lessresistant to laser treatment. In addition, it is very difficult to forman antireflection film for ArF and F₂ lasers having emission wavelengthsof 193 nm and 157 nm, respectively, which are deep ultraviolet lightused for lithography, because, in order to form a broadbandantireflection film, at least three layers must be vapor-deposited.

[0010] Furthermore, once a film is damaged, the substrate having thedamaged film needs to be restored through two processes, rough polishingand ultra-fine polishing.

[0011] This is because the film formed by the known vapor depositionlocally has, between the surface of the substrate and the deposited thinfilm, an absorption layer which cannot be removed by ultrasonic cleaningor laser cleaning, that is, laser light exposure, and because theremaining absorption layer is turned into plasma by a high-energy laserlight to destruct the deposited film.

[0012] A broadband antireflection film is formed by vapor-depositing twomaterials having a high refractive index and a low refractive index in 3to 7 layers, or by depositing two materials in 100 to 300 layers eachhaving a thickness of one hundredth to one three hundredth that of amonolayer film. However, these methods increase manufacturing costs incomparison with a method for forming a monolayer film.

[0013] Also, when a damaged film is removed from a substrate to reusethe substrate, it takes at least about 5 hours to restore the substratebecause of the two polishing processes.

[0014] The method proposed by the inventors, using the mixed film andthe chemical process to form a porous thin film whose refractive indexhas a gradient is applicable to oxide, but the method is difficult toapply to fluoride. In addition, two materials must be vapor-depositedwhile the mixing ratio of the materials is varied in the process offorming the mixed film. However, this makes it difficult to control thedeposition rate and thus makes it difficult to steadily obtain a desiredreflectance.

[0015] The porous MgF₂ and CaF₂ films formed by the sol-gel processesproposed by Milam or Thomas have a reflectance of 0.5% or less at apredetermined wavelength and a laser resistance two or more times thanthat of a thin film formed by vapor deposition. However, the surfacesthereof undesirably exhibit a low mechanical strength.

[0016] This is because the porous thin films are formed of colloidalparticles adhered onto the surface of a substrate by Van der Waals forceand therefore easily peel if an external mechanical force is applied tothe thin film.

[0017] Accordingly, the present invention is intended to solve theseproblems and the object of the present invention is to provide a methodfor forming an optical thin film used as an optical element used forlaser systems including high-energy lasers and an optical element usedfor optical apparatuses. The optical film has a high laser resistanceand a reasonable hardness at the surface thereof, and can be used with adesired refractive index gradient in a broad wavelength band from deepultraviolet to infrared. In particular, the method provides a porousfluoride thin film for preventing reflection in a deep ultravioletregion, easily vapor-deposited on a desired substrate with goodreproductively. Also, the thin film can be easily removed to reuse thesubstrate if the film is damaged.

[0018] In order to achieve the object, the present invention providesthe following:

[0019] (1) A method for forming an optical thin film comprises the stepsof: vapor-depositing a water-insoluble material for preventingreflection onto an optical element substrate; vapor-depositing awater-soluble material having a higher particle energy onto the surfaceof the water-insoluble material; allowing the water-soluble material topermeate deep into the water-insoluble material to form a mixed film onthe surface of the substrate; and subsequently dissolving and removingthe water-soluble material to form a porous thin film comprising thewater-insoluble material.

[0020] (2) In the method for forming an optical thin film described in(1), the optical element substrate comprises one material selected fromthe group consisting of: optical glasses including quartz glass,borosilicate crown glass, and phosphate glass; crystals includingfluorite, rock crystal, and sapphire; laser crystals including YAG andAl₂O₃; ceramics; semiconductors; plastics; and metals.

[0021] (3) In the method for forming an optical thin film described in(1), the water-insoluble material comprises one compound selected fromthe group consisting of: oxides including silica; and fluoridesincluding magnesium fluoride.

[0022] (4) In the method for forming an optical thin film described in(3), the oxides and the fluorides consist of SiO₂, Al₂O₃, CeO₂, HfO₂,Ta₂O₅, ThO₂, TiO₂, ZrO₂, Sc₂O₃, MgF₂, AlF₃, CaF₂, LiF, LaF₃, PbF₂, andNdF₃.

[0023] (5) In the method for forming an optical thin film described in(1), the water-soluble material comprises one compound selected from thegroup consisting of fluorides, oxides, chlorides, and phosphates.

[0024] (6) In the method for forming an optical thin film described in(5), the fluorides, the oxides, the chlorides, and the phosphatesconsist of NaF, Na₃AlF₆, LiF, B₂O₃, MgCl₂, NaCl, NiCl₂, LaCl₃, LiCl, andNaPO₃.

[0025] (7) The method for forming an optical thin film described in (1)further comprises the step of forming an overcoat film on the surface ofthe porous thin film.

[0026] (8) In the method for forming an optical thin film described in(7), the overcoat film comprises a fluoride or an oxide and is 50 to 500Å in thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a schematic illustration showing a method for forming anoptical thin film according to the present invention.

[0028]FIG. 2 is a graph showing the transmittance of an optical thinfilm according to the present invention with respect to wavelengths.

[0029]FIG. 3 is a sectional view of an overcoated optical thin filmaccording to a sixth example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0030] The embodiment of the present invention will now be illustratedin detail.

[0031]FIG. 1 is a schematic illustration showing a method for forming anoptical thin film according to the present invention.

[0032] First, as shown in FIG. 1(a), a water-insoluble material 2 forpreventing reflection is vapor-deposited onto the surface of an opticalelement substrate 1, and then a water-soluble material 3 isvapor-deposited which includes energetic particles capable of permeatinginto the first water-insoluble layer in a sufficient amount to form amixed film on the optical element substrate 1. Reference numeral 4designates an air space. Referring to FIG. 1(b), the water-solublematerial 3 in the mixed film is removed to form a porous thin film 5 ofthe water-insoluble material on the optical element substrate 1.Reference numeral 6 designates an air space.

[0033] The water-insoluble material deposited to form the mixed film maybe an oxide, such as silica (SiO₂), or a fluoride, such as magnesiumfluoride (MgF₂), and the water-soluble material may be a fluoride, suchas sodium fluoride (NaF), an oxide, a chloride, or a phosphate. But,instead of those materials, other materials having the same function maybe used. Such water-insoluble materials include Al₂O₃, CeO₂, HfO₂,Ta₂O₅, ThO₂, TiO₂, ZrO₂, Sc₂O₃, MgF₂, AlF₃, CaF₂, LiF, LaF₃, PbF₂, andNdF₃, and such water-soluble materials include Na₃AlF₆, LiF, B₂O₃,MgCl₂, NaCl, NiCl₂, LaCl₃, LiCl, and NaPO₃.

[0034] The optical element substrate 1 is generally formed of an opticalglass, such as quartz glass, borosilicate crown glass (BK-7), andphosphate glass. Alternatively, crystals such as fluorite (CaF₂), rockcrystal (SiO₂), and sapphire (AlO₃), laser crystals such as YAG andAl₂O₃, ceramics, semiconductors, plastics, and metals may be used.

[0035] Also, the vapor deposition includes vacuum deposition,sputtering, ion plating, chemical vapor deposition, and theircombinations. However, it is important that the deposition apparatus andtechnique used here are designed to allow the water-soluble material topermeate deep into the water-insoluble layer.

[0036] The water-soluble material in the deposited mixed film,constituted of the above-described material, such as NaF, and Na₃AlF₆,is easily removed using pure water or ultra-pure water, and thus theremaining water-insoluble SiO₂, MgF₂, or the like forms a porous thinfilm 5 having a refractive index gradient on the substrate 1.

[0037] The refractive index n_(p) and refractive index gradient of theresulting porous thin film 5 at the air space 6 side can be changed byvarying the amount and depth of the water-soluble material 3 permeatinginto the water-insoluble material 2.

[0038] Characters n₁, n₂, and n_(s) in FIG. 1(a) designate therefractive index of the water-soluble layer 3, the refractive index ofthe water-insoluble layer 2, and the refractive index of the substrate1, respectively; and character n_(p) in FIG. 1(b) designates therefractive index of a thin layer between the air space 6 and the porousthin film 5.

[0039] The optical thickness of the porous thin film is defined by theproduct of the refractive index and thickness of the porous thin filmand is set at one fourth of a specific wavelength, that is, λ/4 (λ: thewavelength of specific incident light).

[0040] When the refractive index of the porous thin film 5 is set lowerthan that of the substrate 1 and the optical thickness is set at an oddnumber time of λ/4, the resulting antireflection film has a smallestreflectance for specific incident light. Also, the porous thin film isnonuniform film having a refractive index gradient. Therefore, theresulting antireflection film can be used in a highly broad wavelengthband from deep ultraviolet to infrared.

[0041] Also, the reflectance of the porous thin film 5 can be muchreduced in comparison with that of a thin film formed by conventionalvacuum deposition.

[0042] For example, the refractive index of a uniform monolayer thinfilm formed by conventional vacuum deposition is no less than about1.38, and, in this instance, the reflectance of this film in combinationwith a quartz glass substrate is 1.8%. In contrast, the porous thin film5 according to the present invention has a reflectance as small as0.25%, as shown in the embodiment.

[0043] In this connection, the reflectance of the porous thin filmaccording to the present invention is expressed by the followingequations.

[0044] Reflectance R₁ of vertical incident light when only thewater-insoluble material 2 is vapor-deposited at a thickness of onefourth the wavelength on a substrate in FIG. 1(a) is expressed byequation (1): $\begin{matrix}{R_{1} = \left( \frac{n_{2}^{2} - n_{s}}{n_{2}^{2} + n_{s}} \right)^{2}} & (1)\end{matrix}$

[0045] Reflectance R_(p) in the case where there is a refractive indexgradient, as shown in FIG. 1(b), is expressed by: $\begin{matrix}{{{R_{P} \approx {1 - {\frac{4n_{p}n_{2}n_{s}}{\left\lbrack {{n_{p}n_{g}} + n_{s}} \right\rbrack^{2} - {{\left\lbrack {n_{p}^{2} - 1} \right\rbrack \left\lbrack {n_{s}^{2} - n_{2}^{2}} \right\rbrack}\quad \sin^{2}\quad \left( {\theta/2} \right)}}\quad {where}}}}\quad {\theta = {{\frac{4\pi}{\lambda}{\int_{0}^{d_{p}}{{n(z)}\quad {z}}}} \approx {{\frac{2\pi}{\lambda}\left\lbrack {n_{p} + n_{2}} \right\rbrack}d_{p}}}}}\quad} & (2)\end{matrix}$

[0046] λ and d_(p) represent the light wavelength and the thickness ofthe porous thin film, respectively.

[0047] A first example of the present invention will now be described.

[0048] MgF₂ was vapor-deposited at a thickness of about 60 nm onto thesurface of a quartz glass substrate having a diameter of 40 mm which wasnot heated, by resistance heating. Next, the pressure of the depositionbath was set at 0.7 mTorr by introducing argon gas, and subsequently NaFwas deposited at a thickness of 20 nm on the MgF₂ layer by ion plating,followed by heating at 200° C. for 10 minutes.

[0049] The resulting two-layered film was immersed in ultra-pure waterof 25° C. for 1 minute to remove the NaF layer. FIG. 2 shows the changesin transmittance of the resulting porous MgF₂ thin film formed on onesurface of the quartz glass substrate.

[0050] The reflectance of the porous MgF₂ thin film was 0.25% at awavelength of 332 nm. For information, the reflectance of the surface ofthe quartz substrate is 3.75% at the wavelength of 332 nm, and thereflectance in the case where MgF₂ is deposited on the quartz glasssubstrate at a thickness of one fourth the wavelength is 1.8%. Hence,the reflectance of the porous thin film is reduced to about one sevenththe reflectance of a common MgF₂ monolayer film. The surface roughnessof the porous MgF₂ film was 12 Å (RMS) As for stress, while the tensilestress of a mixed film is 483 kgf/cm², the stress of the porous MgF₂thin film from which NaF was removed by immersing the film in ultra-purewater was extremely reduced to 48 kgf/cm². The mechanical strength ofthe film surface is about twice that of a metal thin film; hence thereis no problem in practice.

[0051] A second example of the present invention will now be described.

[0052] SiO₂ was vapor-deposited at a thickness of about 68 nm onto thesurface of a quartz glass substrate having a diameter of 40 mm which wasnot heated, by an electron beam technique. Then, a porous SiO₂ film wasformed on one surface of the quartz glass substrate in an identicalmanner to the first example. The reflectance of the resulting porousSiO₂ film was 0.5% at a wavelength of 370 nm. This film had a surfaceroughness of 10 Å (RMS) and a mechanical strength as large as that ofthe porous MgF₂ film.

[0053] A third example of the present invention will now be described.

[0054] A porous MgF₂ film was formed to a thickness of 48 nm on thesurface of a CaF₂ crystal substrate having a diameter of 40 mm, in anidentical manner to the first example. The reflectance of this film was0.8% at a wavelength of 270 nm. While the reflectance of the CaF₂crystal substrate is 3.57% at the wavelength of 270 nm, the reflectancein the case where MgF₂ is vapor-deposited on the CaF₂ substrate at athickness of one fourth the wavelength is 2%. Hence, the reflectance ofthe porous MgF₂ film on the CaF₂ substrate was reduced to about{fraction (1/2.5)} the reflectance of a conventional MgF₂ monolayerfilm.

[0055] A fourth example of the present invention will now be described.

[0056] First, a MgF₂ layer, acting as the water-insoluble material, wasvapor-deposited onto a quartz glass substrate having a diameter of 40mm, and a Na₃AlF₆ layer was formed to a thickness of 20 nm on the MgF₂layer by ion plating. After heating the substrate as in the firstexample, Na₃AlF₆ was removed using ultra-pure water to form a porousMgF₂ thin film. The optical and mechanical performances of the resultingthin film were as great as those in the first example.

[0057] A fifth example of the present invention will now be described.

[0058] The porous MgF₂ film formed in the first example was opticallypolished with a planetary grinder using a CeO₂ suspension. As a result,the substrate was able to be restored to have an ultra-smooth surface ina short period of time as little as about 10 minutes without beingdamaged. A mew porous MgF₂ film can be formed again on the surface ofthe restored substrate, and the optical performance did not changed.

[0059]FIG. 2 shows that the porous MgF₂ film has a reflectance of 0.25%at the wavelength of 332 nm and, thus, can prevent reflection in a broadwavelength band.

[0060] A sixth example of the present invention will now be described.

[0061]FIG. 3 is a sectional view of an optical thin film of a sixthexample in which the surface of a porous thin film formed in theforegoing examples is overcoated.

[0062] In this drawing, reference numerals 1, 2, and 3 designate a glasssubstrate, the above-described porous thin film formed on the substrate1, and an overcoat film formed on the porous thin film 2, respectively.

[0063] A fluoride layer (such as a MgF₂ layer) or an oxide layer (suchas a SiO₂ layer) was overcoated at a thickness of 50 to 500 Å on theporous thin film 2 on the glass substrate 1. Overcoating may beperformed by resistance heating, electron beam heating, ion beamsputtering, or the like.

[0064] As a result, a porous thin film covered with the overcoat filmused for a wavelength of 266 nm was obtained.

[0065] While a simple porous thin film has a reflectance of, forexample, 0.25% at a wavelength of 200 nm, the porous thin film on whichMgF₂ layer having a thickness of about 80 A was overcoated had areflectance of 0.2% at a wavelength of 266 nm. By overcoating a film,the thickness was increased and, consequently, the wavelength at whichthe transmittance becomes maximum (the reflectance is minimum) wasshifted toward the longer wavelength side, from 200 to 266 nm.

[0066] The mechanical strength was also enhanced to a degreesubstantially equivalent to the strength of conventional thin films.

[0067] Also, the deterioration with time (deterioration with age) of thereflectance can be suppressed.

[0068] For example, (1) the reflectance of a simple porous thin film(for 266 nm) is 0.25% before use, and is changed to about 1.2% after oneyear has elapsed. In contrast, (2) the reflectance of the overcoatedporous thin film (for 266 nm) is 0.2% before use, and is only slightlychanged to about 0.4% even after one year has elapsed. Thus thedeterioration with age can be suppressed.

[0069] By forming an overcoat film on the surface of the porous thinfilm, three properties of the resulting film, that is, the mechanicalstrength, the deterioration of the film reflectance with age, and thefilm reflectance, can be improved.

[0070] Since the porous thin film is porous and hence has a low density,it has a refractive index gradient. Consequently, it can easily be anantireflection film even if it is a monolayer film. However, the densityof the porous thin film gradually increases and thus the reflectancedisadvantageously increases because the porous thin film has many pores(the pore size is the same as that of molecules) into which water orparticles (dust) in the air having a size of molecules are trapped.However, according to the present example, this disadvantage can beovercome.

[0071] The present invention is not limited to the above-describedexamples, and various modifications may be made according to the spiritof the invention, without departing from the scope of the invention.

[0072] As described above in detail, the present invention has thefollowing advantages.

[0073] (1) By removing the second layer, or the water-soluble material,a porous thin film having a refractive index gradient can be easilyformed and, thus, an antireflection film having an extremely smallreflectance can be achieved which could not be obtained by the knownvacuum deposition.

[0074] (2) By changing the first layer material, or the water-insolublematerial containing oxide or fluoride, various optical thin films havingvarious refractive index gradients can be formed with goodreproducibility. Thus an optical thin film having small reflectance in abroad wavelength band from deep ultraviolet to infrared can be achievedwhich could not be obtained in a monolayer film formed by the knownvacuum deposition.

[0075] (3) The pores of the porous thin film prevent laser exposure fromincreasing the pressure of the absorption material in the thin film andthus prevent the laser exposure from damaging the optical thin film. Asa result, the resulting thin film has a laser resistance and thus can beused for a high-energy leaser.

[0076] (4) Since the adhesion of the porous thin film to the substrateis lower than that of a conventional hardly coated optical thin film, adamaged thin film can be easily removed using an optical polishingliquid containing ultrafine particles suspended, in a short period oftime without affecting the surface of the substrate.

[0077] (5) By forming an overcoat film on the surface of the porous thinfilm, three properties of the resulting film, that is, the mechanicalstrength, the deterioration of the film reflectance with age, and thefilm reflectance, can be improved.

INDUSTRIAL APPLICABILITY

[0078] The resulting optical thin film of the present invention issuitably used for an optical element of laser systems includinghigh-energy lasers, an optical element of optical apparatuses, such asdigital cameras, video cameras, and liquid crystal projectors, and aprotective film for solar cells, pictures, and displays.

1. A method for forming an optical thin film, comprising the steps of:(a) vapor-depositing a water-insoluble material for preventingreflection onto an optical element substrate; (b) vapor-depositing awater-soluble material having a higher particle energy onto the surfaceof the water-insoluble material; (c) allowing the water-soluble materialto permeate deep into the water-insoluble material to form a mixed filmon the surface of the substrate; and (d) subsequently dissolving andremoving the water-soluble material to form a porous thin filmcomprising the water-insoluble material.
 2. A method for forming anoptical thin film according to claim 1, wherein the optical elementsubstrate comprises one material selected from the group consisting of:optical glasses including quartz glass, borosilicate crown glass, andphosphate glass; crystals including fluorite, rock crystal, andsapphire; laser crystals including YAG and Al₂O₃; ceramics;semiconductors; plastics; and metals.
 3. A method for forming an opticalthin film according to claim 1, wherein the water-insoluble materialcomprises one compound selected from the group consisting of: oxidesincluding silica; and fluorides including magnesium fluoride.
 4. Amethod for forming an optical thin film according to claim 3, whereinthe oxides and the fluorides consist of SiO₂, Al₂O₃, CeO₂, HfO₂, Ta₂O₅,ThO₂, TiO₂, ZrO₂, Sc₂O₃, MgF₂, AlF₃, CaF₂, LiF, LaF₃, PbF₂, and NdF₃. 5.A method for forming an optical thin film according to claim 1, whereinthe water-soluble material comprises one compound selected from thegroup consisting of fluorides, oxides, chlorides, and phosphates.
 6. Amethod for forming an optical thin film according to claim 5, whereinthe fluorides, the oxides, the chlorides, and the phosphates consist ofNaF, Na₃AlF₆, LiF, B₂O₃, MgCl₂, NaCl, NiCl₂, LaCl₃, LiCl, and NaPO₃. 7.A method for forming an optical thin film according to claim 1, furthercomprising the step of forming an overcoat film on the surface of theporous thin film.
 8. A method for forming an optical thin film accordingto claim 7, wherein the overcoat film comprises a fluoride or an oxideand is 50 to 500 Å in thickness.